As is well known, the viscosity of lubricating oils varies with the temperature. Many oils must be employed over a wide temperature range, and it is important that the oil not be too viscous at low temperatures nor too thin at high temperatures. Variation of the viscosity-temperature relationship of an oil is indicated by the well-known viscosity index value. The higher the viscosity index, the less the change in viscosity with change in temperature. At present, viscosity at 210.degree. F. and at 100.degree. F. is used to determine the viscosity index (VI).
Many attempts have been made to improve the properties, particularly the viscosity-temperature relationships and shear stability of lubricating oils. It has been proposed to add various materials to the lubricating oils fot this purpose, among those materials being various polymers. See generally U.S. Pat. Nos. 3,554,911; 3,668,125; 3,772,196; 3,775,329; and 3,835,053. These polymers, disclosed in the above patents, are generally hydrogenated substantially linear polymers of conjugated dienes and, optionally, monoalkenyl aromatic compounds. Such hydrogenated polymers are typically prepared by the anionic solution polymerization of the monomers followed by hydrogenation. This process comprises polymerizing a conjugated diene and, optionally, a monoalkenyl aromatic compound, in solution, in the presence of an anionic initiator to form an unsaturated so-called living polymer. Examples of hydrogenated substantially linear polymers which are commercially used as oil additives include hydrogenated styrene/butadiene and hydrogenated styrene/isoprene copolymers.
Insofar as hydrogenated substantially linear polymers are concerned it is possible to increase the thickening power, and therefore possible to use a lower amount in lubricating oil compositions, by increasing their molecular weight. However, this has the disadvantage that the shear stability of such polymers, in general, decreases with increasing molecular weight. Consequently, the choice of any particular polymer is usually a compromise between a high amount of a lower molecular weight polymer having a good shear stability and a low amount of a higher molecular weight polymer having a poor shear stability. In addition it is difficult to prepare oil concentrates of such polymers containing more than about 10%w thereof which is a further disadvantage of such polymers.
It is significant to note that none of the three prior art patents which disclose a possible branch structure for the polymer (U.S. Pat. Nos. 3,668,125; 3,775,329; and 3,835,053) actually employ a branched polymer in any of their examples. Further, none of these three references in describing the preparation of the polymer VI improvers disclose a coupling agent that would result in polymer having greater than four branches.
A new polymer has now been found that overcomes many of the deficiencies found in the polymers described in the above patents.